1. Field of the Invention
The present invention relates to nearly spherical graphite particles suitably used as an anode material for lithium secondary battery, a separator material for fuel cell, etc., as well as to a process for production thereof.
2. Description of the Prior Art
Graphite particles have found wide applications as an anode material for lithium secondary battery, a separator material for fuel cell, etc. The raw materials for the graphite particles used in such applications are natural graphite and artificial graphite.
Graphite particles obtained by grinding natural graphite show striking anisotropy in particle shape, owing to the crystal structure. The particle shape of such graphite particles is scaly (plate-like). That is, the crystals of the graphite particles have a structure constituted by a number of large AB planes laminated in the C axis direction. Since each AB plane has a large area while the laminate thickness in the C axis direction is small, the particle shape is plate-like as a whole.
The anode of lithium secondary battery is generally constituted by a collector (e.g. a copper foil) and a thin graphite layer covering the surface of the collector. In order to allow a lithium secondary battery to have large charge and discharge capacities, the graphite layer is preferred to have a high density. Therefore, the graphite layer is generally compressed using a press, a roll or the like to obtain a higher density.
When, in producing an anode using graphite particles obtained from natural graphite, the graphite layer on the collector is compressed using a press, a roll or the like, however, the graphite particles of the graphite layer are orientated so that each AB plane of the graphite particles becomes parallel to the compressed surface of the graphite layer because each graphite particle is a thin scale (a thin plate).
That is, each plate-like graphite particle constituting the graphite layer is orientated so that its AB planes become parallel to the surface of the collector. Hereinafter, such orientation of graphite particles in graphite layer is called simply “orientation”.
Orientation of graphite particles in the anode of battery is not preferred. The reasons are as follows. Firstly, since the electrolytic solution in battery is unable to pass thorough each AB planes of graphite particles, the electrolytic solution penetrates hardly into the graphite layer of anode wherein the graphite particles are orientated. As a result, the contact between graphite and electrolytic solution is restricted only to the surface of the graphite layer or its vicinity. Next, the conductivity of graphite crystals is large within the AB plane and small in the C axis direction. Meanwhile, the direction of flow of electricity in the graphite layer is the thickness direction of the graphite layer. This direction agrees to the C axis direction of the orientated graphite particles. Therefore, the electric resistance of the anode becomes large. As a result, the charge and discharge capacities of the battery become small.
The separator of fuel cell is produced by molding a mixture of particulate graphite and a resin into a plate shape using a press. The main function of the separator is to separate the flow of a fuel gas and the flow of an oxygen-containing gas from each other so as to prevent the mixing of the two gases. The separator also has a function as a collector; in this case, electricity flows in the separator in its thickness direction. Therefore, appearance of orientation in the separator is not preferred as in the anode material for lithium secondary battery.
Also in the graphite electrodes used for other applications, orientation appears often.
Artificial graphite can be produced as nearly spherical particles, depending upon the production process. It can also be produced as particles low in anisotropy. Artificial graphite can be produced, for example, as spherical graphite particles having a structure in which disc-shaped graphite plates different in radius are laminated in the C axis direction, or as columnar graphite particles having a structure in which disc-shaped graphite plates equal in radius are laminated in the C axis direction.
Such artificial graphite, however, is generally expensive and low in crystallinity. Use of artificial graphite low in crystallinity as an anode material is not preferred because such graphite is small in charge and discharge capacities per unit mass of graphite.
Use of artificial graphite low in crystallinity as a material for separator is not preferred, either, because such graphite has low conductivity.
Meanwhile, artificial graphite high in crystallinity has properties close to those of natural graphite. Therefore, artificial graphite high in crystallinity, when ground, becomes scaly particles as in the case of natural graphite.